The present invention relates to signal frequency conversion.
Mobile cellular telephones communicate using radio frequency signals. Voice data input into a mobile cellular telephone microphone is a low frequency or baseband signal. A transmitter converts the baseband signal to a radio frequency signal for transmission by the mobile cellular telephone. The baseband signal is passed through a low frequency stage, converted to a radio frequency, and then passed through a high frequency stage. In one transmitter, a Cartesian approach to processing the baseband signal is used. Two low pass filters receive and filter baseband inphase (I) and quadrature (Q) signals. Baseband variable gain amplifiers scale the filtered I and Q signals. Two class A gilbert cell mixers up-convert the scaled I and Q signals to radio frequency I and Q signals. The radio frequency I and Q signals are summed into a single radio frequency signal. The summed radio frequency signal is then scaled by a class AB radio frequency variable gain amplifier. The scaled high frequency signal is finally passed to an antenna for transmission.
The transmitter incorporates baseband and radio frequency gain control for tradeoff between linearity, noise, local oscillator feedthrough, I/Q balance, and power consumption. For example, an existing transmitter may sacrifice power consumption to gain linearity. Both the low frequency stage and the high frequency stage include class A or class AB devices. In mobile cellular telephones, current and expected standards demand low noise, low current, high linearity, and more than 40 dB of power control.